Debris-flow valleys in Southeast Tibet were found distinct in their channel structure.The source area of debris flow usually extends to the middle reaches,with cross and confluence of the channels.The traditional method based on the full-valley parameters is inadequate to provide accurate parameters for engineering designs.We proposed a new method to calculate the peak discharge of debris flow.Based on field survey in Shannan Prefecture of Tibet,a traditional valley was measured.We calculated the hydrograph with the triangle model and concentration time of sub-basins with large areas(25% of full-valley area),and then obtained the converged hydrograph and corresponding peak discharge,which is amended by the concentration times of two sub-basins.Comparing the results of the full-valley method and the proposed new method,we concluded that the proposed method accurately captured the contribution of dynamic sub-basin discharges and is of greater applicability.